Emphasis is being placed on elucidating the nature of the cell-virus association in latent infection with the parvovirus, H-1. The question of viral integration into the host cell genome is being examined thoroughly since it would represent the unique situation in animal virology of a non-oncogeic virus undergoing genomic integration. The consequences of the latent infection for the phenotype, including ultrastructure, of the carrier cell will be pursued. Activation of the latent infection by superinfection with helper virus will give an opportunity to examine the interaction of the latent viral genome with oncogenic and nononcogenic helpers. The role of host cell modification of the virion will be further examined in relation to viral density, HA and infectivity, and to the structure of VP2, the major virion protein. Immunoelectronmicrographic techniques will be used to trace the sites of accumulation of virion proteins and their subsequent assembly in the nucleus. In addition, elucidation of the unusually selective effects of halogenated pyrimidines on H-1 replication will be attempted, including effects on genome transcription and vira message translation. In vitro DNA synthesis comparing host cell and viral DNA synthesis will be further explored.